Process for producing a heterogenous penetration-bonded metal

ABSTRACT

The invention relates to a method of producing a heterogenous, penetration-bonded metal for use as a contact material for vacuum switches. The pores of a pore containing sintered structure comprising a burn-off resistant, high melting metal such as tungsten, rhenium or molybdenum, are filled with a lower melting metal of good electrical conductivity, such as silver, copper, or a lower melting alloy of these metals. A structure or skeleton of the high melting metal is first sintered. This is then saturated or impregnated with the lower melting metal or alloy. Subsequently, at least one metal, with a high vapor pressure is installed as an alloy component, by means of diffusion, into the lower melting impregnation metal which is contained in the pores of the high melting metal structure. The diffusion metal is located in an auxiliary metal and the diffusion temperature is below the melting temperature of the impregnation metal.

United States Patent Schreiner et al.

{45]March 20, 1973 1 PROCESS FOR PRODUCING A HETEROGENOUS PENETRATION- BONDED METAL [75] lnventors: Horst Schreiner, Numberg; Heinrich Hassler, Wendelstein, both of Germany [73] Assignee: Siemens Aktiengesellschaft, Berlin and Munich, Germany [22] Filed: March 25, 1971 [21] App1.No.: 128,059

[30] Foreign Application Priority Data March 26, 1970 Germany ..P 20 14 639.8

[52] 'U.S. Cl ..75/208 R, 29/1821, 75/200 [51] Int. Cl. ..B22t 3/26, 1322f 7/02 [58] Field of Search ..117/217; 75/200, 208 R;

[56] References Cited UNITED STATES PATENTS 3,069,757 12/1962 Beggs et al ..29/182.1

3,407,061 10/1968 Hutkin ..75/208 R 2,379,232 6/1945 l-lensel ..29/l82.l

3,290,124 12/1966 l-loltzclaw, Jr.. .....75/200 X 3,305,324 2/1967 Krock et a1 ..29/l82.l

3,318,696 5/1967 Krock et a1. ..75/208 R 3,338,687 8/1967 Dickinson et al ..29/l82.l

3,360,348 12/1967 Schreiner ..75/208 R X 3,366,463 1/1968 Schreiner ..75/208 R X 3,393,056 7/1968 Zdanuk et al. ..29/182. 1 3,489,530 l/l970 Schreiner ..75/208 R X [5 7 ABSTRACT The invention relates to a method of producing a heterogenous, penetration-bonded metal for use as a contact material for vacuum switches. The pores of a pore containing sintered structure comprising a burnoff resistant, high melting metal such as tungsten, rhenium or molybdenum, are filled with a lower melting metal of good electrical conductivity, such as silver, copper, or a lower melting alloy of these metals. A structure or skeleton of the high melting metal is first sintered. This is then saturated or impregnated with the lower melting metal or alloy. Subsequently, at least one metal, with a high vapor pressure is installed as an alloy component, by means of diffusion, into the lower melting impregnation metal which is contained in the pores of the high melting metal structure. The diffusion metal is located in an auxiliary metal and the diffusion temperature is below the melting temperature of the impregnation metal.

4 Claims, 4 Drawing Figures PROCESS FOR PRODUCING A HETEROGENOUS PENETRATION-BONDED METAL The present invention relates to a method of producing a heterogenous, penetration-bonded metal for use as contact material for vacuum switches. The pores of a pore containing sintered structure, comprising a burnoff resistant, high-melting metal, such as tungsten, rhenium or molybdenum, are filled with a lower melt ing metal with good electrical conductivity, such as silver, and copper, or with a lower melting alloy of these metals.

Contact materials for vacuum switches are required to have, in addition to extremely small gas volume, also a low welding power, a small burn-off, low contact resistance and a low chopping effect. A chopping effect causes the arc to break during-the switching of low currents whereby the inductivity effect produces voltage peaks which may result in breakthroughs. To minimize the chopping effect, a small amount of a metal with high vapor pressure is added to the contact material. This reduces the constriction of the arc caused by current forces. When contact materials comprising WCu or MoCu are manufactured, one encounters the difficulty of distributing, uniformly, an anti-chopping metal, e.g. bismuth across the entire volume of the contact layer. In addition, a contact piece whose surface contains shares of CuBi cannot be soldered impeccably with a carrier metal such as Cu, for example, by the heretofore tested hard solders.

An object of our invention is to provide a method for producing a heterogenous penetration-bonded metal,

to be used as a contact material for vacuum switches, which will make it possible to install and distribute the bismuth in the contact material, only with a defined depth of penetration.

To this end, and in accordance with the invention, a porous or skeleton structure is first of all sintered from a high melting metal (structure metal). This metal has to 60 and preferably 30 40 percent by volume pores which are impregnated with the lower melting metal or with the lower melting metal alloy (impregnation metal). Subsequently, at least one metal with high vapor pressure is installed as an alloying component (diffusion metal) by means of diffusion and in a liquid state, into the lower melting impregnation metal, which is located in the pores of the high melting metal structure. The diffusion metal is installed together with an auxiliary metal and the diffusion temperature is below the melting temperature of the impregnation metal.

The advantage of a two layer contact piece so produced is that a layer free of bismuth is present on the application side. This layer consists, for example, of MoCu or WCu and can be connected in an impeccable manner, with the carrier metal, by conventional hard silver solders.

According to the present invention, the diffusion is effected via a liquid phase which forms from the diffusion metal, the auxiliary metal and the impregnation metal. The auxiliary metal has the task, in addition to forming a liquid phase with the impregnation metal, to dissolve the diffusion metal and to produce the necessary concentration through dilution.

Also, the amount of auxiliary metal is so determined that with the diffusion conditions as determined by temperature and time, a defined diffusion depth is obtained through the formation of a specific amount of liquid phase with the impregnation metal. The right concentration of diffusion metal in the auxiliary metal helps to adjust the desired concentration of diffusion metal in the impregnation metal or alloy, which forms with the auxiliary metal, following the diffusion via the liquid phase. The diffusion with the liquid phase proceeds quickly compared to a diffusion in a solid state, so that a uniform distribution is obtained even after 10 to 30 minutes. The end of diffusion is determined by the equilibrium conditions between auxiliary metal, diffusion metal and impregnation metal at the diffusion temperatures selected. When the liquid phase disappears the diffusion can only be continued in a solid state, wherefore its rate is considerably reduced. Since the diffusion in a liquid state occurs rapidly and uniformly, the depth of penetration is approximately the same over the entire area and an almost planar diffusion surface area occurs.

Another particular advantage of the present invention is the economy and the technical maintenance of the diffusion temperature, within narrow limits in association with the production of such contact pieces. When pure bismuth is used as diffusion metal and MoCu as the structure and impregnation metal, an equilibrium Bi-content of 2 percent results in a diffusion temperature of l075 C. As this temperature is only 8 C below the melting temperature of the impregnation metal copper, it is virtually impossible to comply with the temperature requirement in a furnace for the purpose of manufacture. Similar conditions occur when copper is used as auxiliary metal and bismuth as the diffusion metal and also when the equilibrium requirement of 2% Bi is aimed at in Cu saturation metal. The advantage associated with the use of an auxiliary metal which forms a lower melting alloy with the saturation metal, e.g. mixed crystals, eutectic or peritectic, results in a diffusion temperature which is considerably below the melting temperature of the impregnation metal. The temperature requirement can easily be realized in a furnace used for manufacturing and the diffusion time so selected to last until the equilibrium and the liquid phase disappears.

The practical execution is so effected that the auxiliary metal and the diffusion metal is placed in form of an alloy at a predetermined weight ratio, below or above the contact piece to be diffused, which comprises the structure metal tungsten and the impregnation metal copper.

The use of a pulverulent mixture of the auxiliary metal and the diffusion metal, e.g. silver and bismuth in the form of a powder mixture or a pressed pulverulent mass, is particularly advantageous. In addition to silver and copper, other alloys are also suitable as saturation metals. Suitable diffusion metals are: bismuth, gallium, tellurium and lead; auxiliary metals are: silver, magnesium or silicon, for copper based impregnation. Copper, antimony, silicon and zinc may be used as the auxiliary metal when silver is used as the impregnation metal.

The invention will be described in greater detail with reference to the Drawing and embodiment examples, wherein:

FIGS. 1 to 4 schematically illustrate the method of the invention with reference to the following two Examples:

EXAMPLE 1 This Example describes the product vacuum switch contact piece comprising WCuBi with auxiliary metal as the contact layer with WCu (without bismuth) as the carrier layer.

Tungsten, which was produced by reducing WO with a particle size 60 um, was reduced in hydrogen at l,000 C, for 1 hour. With a pressure of 2.5 lVlp/cm pressed bodies with a diameter of 33 mm and a height of 6 mm were produced whose pressure density 8,, 13.0 g/cm corresponded to a space filling rate r 0.674. The sintering of the pressed bodies ensued at l,600 C during one hour in a vacuum (P 10' Torr). Following the sintering process, the furnace was gassed with argon. The linear sintering shrinkage is l percent.

ln an impregnation device 1 of graphite (FIG. 1) the sintered object 2 was placed upon an impregnating copper metal body 3. As a result, a copper layer of specific thickness was produced on the bottom side of the contact piece being produced. A graphite ring 4 had been placed into the bore of the impregnation device 1.

The impregnation was carried out at l,200 C for a period of 30 minutes, under hydrogen. The hydrogen was subsequently pumped off and evacuation took place for 2 hours at l,300 to l,500 C.

FIG. 2 shows the WCu contact piece 5 in the saturation device 1, after impregnation. The copper layer 6 has formed on the bottom of the contact piece 5. The excess impregnation copper denoted 7, is located on the upper surface. The thickness of the copper layer 6, at the bottom of the copper piece, is usually between 0.5 and mm and preferably between 2 and 5 mm. It

is determined by the height of the graphite ring 4. The excess impregnation copper 7 is subsequently changed to a thickness of about 0.3 mm.

The WCu contact piece 5 with copper layer 6 on the bottom and the turned-up copper layer 8 at the surface (FIG. 3) is coated in the impregnation device 1 with a powder mixture 9 composed of 1.5 g silver and 0.5 g bismuth. The diffusion temperature is thereafter adjusted at l,000 C. A liquid phase occurs out of Cu and Ag with the Bi uniformly distributed therein (in equilibrium about 80% Cu and Ag). in order that the liquid metal does not run down, the copper layer 8 was provided with an elevated edge. The liquid phase causes the formation of a WCuBi layer of uniform thickness. In the example, it amounts to 1.6 mm.

FIG. 4 is a schematic illustration of the layer build-up of the completed contact piece after the CuAgBi layer has been turned off at the surface. The actual contact layer 10 consists of WCuAgBi, under which lies the WCu layer 11 and on the connecting side the Cu layer 12. The copper layer 12 can be hard-soldered perfectly with the carrier member for the contact piece, which is usually made of copper. The hard solder is usually CuAg of eutectic composition.

EXAMPLE 2 This Example describes the production of contact pieces for vacuum switches comprising MoCuBi with auxiliary metals as the contact layer and MoCu (without bismuth) as the carrier layer.

Molybdenum powder which was obtained through a reduction of the oxides and has a particle size 60 um, was reduced in hydrogen at l,00O C for 1 hour. Pressed objects having a diameter of 35 mm and a height of 8 mm were produced with a pressure of l Mp/cm; had a pressure density 8,, 4.96 g/cm, corresponding to a space filling rate R 0.487. The sintering of the pressed objects took place at l,550 C for 1 hour in a vacuum, at a pressure of about 10' Torr. After the sintering process, the furnace is gassed with argon. The linear sinter shrinkage amounts to about 14.5 percent.

The work cycle of the ensuing method was analogous to that in Example 1. impregnation took place at l,200 C for 30 minutes, under hydrogen. Subsequently, the hydrogen was pumped off and evacuation took place for 2 hours, at l,300 to l,500 C. The saturation excess copper is thereafter turned to a thickness of approximately 0.3 mm. The MoCu contact piece with the copper layer on the bottom and the turned-off copper layer on the surface, was coated with a powder mixture composed of 1.5 g silver and 0.5 g bismuth. The copper layer was provided with a raised edge so that the liquid metal could not run off. Thereafter, diffusion took place at temperatures between 900 and 1,000" C. A liquid phase of Cu and Ag developed at 900 to 1,000 C wherein the Bi was uniformly distributed (in equilibrium, the liquid phase has a composition of about 58% Cu and 42% (20%) Ag). The liquid phase forms a MoCuBi layer of uniform thickness. In the disclosed Example it amounts to about 0.1 (1.7) mm, at a diffusion temperature of 900 to l,000 C.

Following the tum-off of the CuAgBi layer at the surface, the layer build-up will be as follows: the actual contact layer consists of MoCuAgBi, below is the MoCu layer,on the connecting side is the Cu layer. The

copper layer may be impeccably soldered with the carrier member for the contact piece, which is usually comprised of copper. The hard solder used is usually CuAg of eutectic composition.

The amount of auxiliary metal in the diffusion zone of the completed contact piece is about 5 to 30, and

preferably 10 to 20 percent by weight of the sum comprising impregnation metal, auxiliary metal and diffusion metal (except structural metal). The diffusion metal in the diffusion zone of the finished contact piece is about 0.3 to 2 percent and preferably 0.4 to 0.8 percent by weight of the sum comprising saturation metal, auxiliary metal and diffusion metal. The depth of diffusion should be within the range of 0.5 to 3 mm. preferably 1.5 to 2.5 mm. A value of 2 to 2.5 mm was obtained with a WCu 20 original body of 30 mm diameter which is diffused, for approximately 15 minutes at 980 C, with a diffusion tablet of 2.2 g and 22 mm diameter of composition AgCu15Te8.

We claim:

1. A method of producing a heterogenous penetration-bonded metal contact, for vacuum switches, said body comprising a porous sintered structure of a burnoff resistant high melting metal selected from tungsten, rhenium and molybdenum, the pores of said sintered structure being filled with a low melting metal with good electrical conductivity selected from silver, copper and mixtures thereof which comprises sintering metal and diffusion metal are together brought into contact with the impregnation metal in the high melting body.

3. The process of claim 2, wherein copper is used as the impregnating metal and the auxiliary metal is selected from silver, magnesium or silicon.

4. The process of claim 1, wherein silver is used as the impregnating metal and the auxiliary metal is selected from copper, antimony, silicon, and zinc. 

2. The process of claim 1, wherein the auxiliary metal and diffusion metal are together brought into contact with the impregnation metal in the high melting body.
 3. The process of claim 2, wherein copper is used as the impregnating metal and the auxiliary metal is selected from silver, magnesium or silicon.
 4. The process of claim 1, wherein silver is used as the impregnating metal and the auxiliary metal is selected from copper, antimony, silicon, and zinc. 